Ohm readings on THHN

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Ok 1m per volt makes more sense. Now is that to ground or phase to phase? I assume on a 480 volt Y 277m would be good. PS a 480 volt motor does not see 480V on the coils but would still see 277v to ground.
 
electricalperson said:
thats perfectly good cable right there you want readings like that. did you megger to ground? and to eachother?
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Each phase and the insulated EGC to the EMT and each conductor to the other.

The thing I thought odd was each measurement was identical which to me suggested operator error.
 
iwire said:
Each phase and the insulated EGC to the EMT and each conductor to the other.

The thing I thought odd was each measurement was identical which to me suggested operator error.
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Just meant they were all good to go and I would expect that reading on a new installation. I get that kind of reading on 200+ feet of 14 to 10 jammed into a conduit that I know is full of water and has been for years.
 
Well just a little input the rule is 1000 volts at 1 megg that has been the norm in most books or schooling on anything electrcal wire , motor windings. trans,panel gutts buss ect ect. But most think your megg reading must be 999 infinity or in the high 500 megg ,this is a misconception most bus bars SQ D- gear 25 megg ohms and buss duck just general speaking 100 megg ohms length is only a slight issue in the megg range unless you have a bad connection . If you test new cable or wire in the morning you get a lower reading then if you test at sundown , if you dry up the conductor or buss bars your reading will go up . ive seen 500 mcm with A=350 megg B=400 megg C= 25 megg on phases most would pull this cable out or think it was bad . Turn it on let it load after 8 hours shut down retest or clean ends or re cut ends dry up the ends . You will get above 800 megg or better 999 why its just dampness between jacket or soap water . Most bad megg readings not less than 5 megg ohms i say are good its just a simple fix . comments ?
 
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Please note that Bob said 2.2 GigaOhms with a "G". That's 2,200 MegaOhms. Plenty of resistance. They are probably all the same because this is the highest value your tester will measure.

Mark
 
bob if you look closely you might see a greater-than symbol (>) in front of the 2.2G. that means your meter has reached the limit of its testing capacity.
we get those readings all the time on our fluke meters on new installations with clean dry wire



CIECO sorry i thought you were someone else ive worked for in arkansas
 
i have been told that one MegOhm per volt applied is a good number to use

that doesnt mean volts applied during the test, it means if you are going to Apply 120 volts to that wire, then 120 MegOhms of insulation resistance is the minimum that is necessary to operate without a fault.

obviously that is a low number, but it Will work. I think people are so used to seeing really high numbers on tests for so long that a low number like 250 megohms seems like something is wrong.
 
2.2G is basically an infinity reading meaning nothing is touching and the air is dri. That is what you should have on a new install, even with wire lube. But a pin hole in the insulation and some wire lub to ground will bring it down big time.
 
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CIECO said:
Ok 1m per volt makes more sense. Now is that to ground or phase to phase? I assume on a 480 volt Y 277m would be good. PS a 480 volt motor does not see 480V on the coils but would still see 277v to ground.
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A 480 volt motor does see 480v on the coils! The windings are tied together, and you have 480v on each phase!
Your original question: Acceptable 500 MCM cable insulation resistance. ---Depends on whether it is new installation or old, and the voltage rating of the cable.
I have meggered thousands of new cable on new construction. If you are talking 600v cable on 500 MCM then engineering would accept 100 megohms, phase to phase and phase to ground. That reading is taken with a megger that is set on the 500 volt output - not with a fluke that would probably read that same cable in the gigohms. Engineering would never accept a megger reading taken with a fluke. I am definitely not bad mouthing flukes - I have three 87's, a 73, and a 741B, and also a Biddle megger with 100v, 250v 500v and 1000v settings. Old cable, depending on the environment will undoubtedly megger lower, but I would not begin worry unless it got to 10 megohms.
If I was representing the client or engineering, and the cable meggered less than 100 meg ohms, with a 500v megger, I would not accept the installation.
It would need to be removed from the conduit, and a new cable pulled.
 
RHJohnson said:
A 480 volt motor does see 480v on the coils! The windings are tied together, and you have 480v on each phase!
Your original question: Acceptable 500 MCM cable insulation resistance. ---Depends on whether it is new installation or old, and the voltage rating of the cable.
I have meggered thousands of new cable on new construction. If you are talking 600v cable on 500 MCM then engineering would accept 100 megohms, phase to phase and phase to ground. That reading is taken with a megger that is set on the 500 volt output - not with a fluke that would probably read that same cable in the gigohms. Engineering would never accept a megger reading taken with a fluke. I am definitely not bad mouthing flukes - I have three 87's, a 73, and a 741B, and also a Biddle megger with 100v, 250v 500v and 1000v settings. Old cable, depending on the environment will undoubtedly megger lower, but I would not begin worry unless it got to 10 megohms.
If I was representing the client or engineering, and the cable meggered less than 100 meg ohms, with a 500v megger, I would not accept the installation.
It would need to be removed from the conduit, and a new cable pulled.
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You just stated that a Fluke megger will read more than ten times higher. This is the first time I've ever read of a Fluke tool being that inaccurate! That's to the point of being dangerous too. Got some documented proof?
 
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